Aim of the experiment:
To estimate the amount of MnO2 present in the given sample of pyrolusite.
Theory
Pyrolusite ore contains a large amount of manganese in the form of manganese dioxide and
little silica. It is a black mineral. Its manganese content in between 60 and 63% silica content
between 10 and 50%. The dissolution of the ore is done in the mixture of conc. HCl and conc.
HNO3. The insoluble residue contains silica (SiO2) and the soluble part contains largely
manganese. Silica is estimated gravimetrically while manganese is estimated volumetrically
using KMnO4. Potassium permanganate is an oxidizing agent in acidic medium and hence can
be used to estimate Mn volumetrically.
The manganese present in the pyrolusite sample is reduced by a known excess of
standard sodium oxalate solution and the unreacted oxalate is titrated against a standardized
solution of KMnO4. Where KMnO4 acts as self-indicator. Pale pink colour is the end point of
the titration.
MnO4 + Na2C2O + 2H2SO4 → MnO4 + Na2SO4 + 2CO2 + 2H2O
(From pyrolusite)
Na2C2O4 + KMnO4 + 8H2SO4 → 2MnSO4 + K2SO4 + 5Na2SO4 + 10CO2 + 8H2O
Apparatus Required
Burette, Pipette, Conical flask, Volumetric flask, Burette stand, Funnel, Beaker.
Chemical Required
Sodium oxalate, KMnO4, H2SO4, Pyrolusite sample and MnO2
Procedure
1. Preparation of standard sodium oxalate sodium
Weight 0.67 gm of sodium oxalate into a clean 100 ml standard flask, dissolve in small amount
of water and make up the solution up to the mark with distilled water. Shake the flask well for
uniform concentration.
2. Preparation of KMnO4 solution
Dissolve 0.79 gm of KMnO4 into a 250 ml standard flask in little distilled water and make up
the solution up to the mark with distilled water shake the flask well for uniform concentration.
3. Preparation of 0.1N H2SO4
Dissolve 0.72 ml of conc. H2SO4 slowly drop by drop in 100 ml of distilled water.
4. Standardization of KMnO4 solution
Rine the burette with KMnO4 fill the burette with KMnO4 up to zero mark, pipetted out 10 ml
of sodium oxalate into a clean 250 ml conical flask and add 5 ml of 0.1N H2SO4 and heat the
solution till fumes come out of the solution (70-80℃). Titrate the hot solution with KMnO4 till
pale pink colour is observed in the solution. Repeat the titration to get concordant value. Let
the titrate value of the sample “X” ml.
5. Standardization of pyrolusite sample
Weight out accurately about 0.20 – 0.25 gm of sample into a 250 ml dry conical flask and add
exactly 40 ml of sodium oxalate with help of pipette and 40 ml of 0.1N H2SO4 solution. Put a
small funnel in the neck of the flask and heat the solution on a small flame. Till all the black
particles of pyrolusite disappear in the conical flask. Now wash the funnel and the side of the
conical flask with little distilled water into the flask and titrate unreacted oxalic acid present in
hot solution with KMnO4 solution taken in the burette. Pale pink colour is the end point of the
titration. Let titre value with unreacted oxalic acid ‘Y’ ml.
Note- The titration of unreacted oxalic must be carried out in hot condition i.e., 60 -70℃.
6. Standardization of blank solution
Pipetted out 40 ml of sodium oxalate solution into a 250 ml conical flask and add 50 ml of
0.1N H2SO4. Keep a small funnel in the neck of the flask and heat 70 -80℃. Rinse the funnel
and neck of the flask with distilled water and titrate the hot solution with KMnO4 till pale pink
coloured end point is obtained. The titre value ‘Z’ ml indicates the total amount of KMnO4
required for the complete reaction of the 40 ml of sodium oxalate.
Calculation
Weight of pyrolusite = 0.25 gm
Normality of KMnO4 = 0.1N
Titrate value of pyrolusite ‘Y’ = 12 ml
Blank titre value ‘Z’ = 21 ml
Volume = Blank titre value (Z) – Titre value of pyrolusite (Y) [ Eq.wt of MnO2 = 43.46 ]
= 21-12 = 09
Amount of MnO2 present in sample = Volume × Normality of KMnO4 × Eq.wt of MnO2 / 1000
= 09 × 0.1N × 43.46
1000
= 0.039
% of MnO2 present in given sample = Amount of MnO2 × 100 / wt. of sample
= 0.039 × 100
0.25
= 15.6 %
Result
The amount of manganese dioxide present in given sample in pyrolusite is 15.6 %.